The present invention relates to a control device for controlling a machine tool which can be used to drive tools, and to a method for safely operating such a machine tool.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
Commercially available machine tools generally have a tool changer which is fitted with a plurality of tools and is used to automatically change the tool needed for the respective necessary machining operation for machining a workpiece. In this case, the tool changer is used to insert the tool currently needed for the machining operation, for example a particular milling cutter, into the tool receiving apparatus of a rotatably arranged spindle. The tool-changing operation and the subsequent machining operation are controlled by a control device of the machine tool. In this case, the control device uses a parts program to control the machining operation and which tool is used for the relevant machining operation. The parts program defines, for example, the speed, in particular the speed of rotation, at which the tool, for example a milling cutter, should rotate.
However, the individual tools often have large differences in mass and large differences in the geometrical structure. A correspondingly large tool, for example a large milling cutter, may generally rotate only at an essentially low speed of rotation in comparison with a small tool, for example a small milling cutter or small drill. If a speed of rotation (the speed of rotation may be in the form of a rotational speed, for example, in this case) which is too high for the tool has now been defined, for example in the parts program, on account of negligence, for example, this may result in severe damage to the tool, to the machine or even, in the extreme case, to people because destroyed parts of the tool, for example, may fly through the area at high speed and may hit people.
Even if the speed of rotation for the respective tool is correct in the parts program, severe damage to the tool, to the machine tool and to people around the machine tool may nevertheless result. In addition to the speed movement limits described above, the tool may often also be accelerated only with a particular maximum acceleration and maximum jerk. If, for example, a tool is accelerated to the accordingly predefined speed of rotation too quickly, it may become detached from the tool receiving apparatus, for example, which may result in the above-mentioned severe damage. This occurs again and again in practice since the control device generally generates the corresponding desired values for controlling the motor, which drives the tool, in such a manner that the desired speed of rotation is reached as quickly as possible, that is to say the tool is accelerated as quickly as possible on the basis of the maximum acceleration capacity of the motor. In the case of small low-mass tools, this procedure is generally optimal but it can result in the abovementioned damage in the case of larger, high-mass or more complex tools.
In commercially available machine tools, it is known practice to monitor the rotational speed of the tool by means of additional hardware which is external to the control system and has an additional measuring system.
It would be desirable and advantageous to address prior art shortcomings and to provide a control device and a method to enable a safe and reliable operation of a machine tool.